A “charged particle column” is used to direct a beam of finely focused charged particles on any sample surface. Such columns are used in tools to irradiate various types of specimen for the purposes of a variety of applications. The following examples relate to columns built for “electrons” but similarly apply to other “columns” that are used to irradiate charged atoms also knows as “ions”.
Scanning electron microscopes (“SEM”) are used gather images of specimens at high magnifications. The beam rasters across a certain area and captures an image. A specific type of SEM, known as a high resolution scanning electron microscopes gather images at very high resolution and low beam current and used to measure dimensions of features on the image, whereas a review scanning electron microscope is used to obtain images at specific locations on the surface of semiconductor wafer used for fabricating integrated circuits already identified by another tool as defects/local abnormalities for the purposes of taking high-resolution images of the defects.
Another type of charged particle tool, known as an e-beam defect inspection tool, is used for localizing “defects” i.e. local abnormalities on the surface of semiconductor wafer used for fabricating integrated circuits.
Another type of charged particle tool, known as an e-beam writer, makes specific patterns on a photoresist layer that has been coated on a semiconductor wafer or a photolithography mask for the purpose patterning these shapes onto an underlying later. A mask writer operates by illuminating a 1st (square) shaping aperture and forming a 1st shape, then deflecting the 1st shape across a 2nd (square) shaping aperture to form a variable rectangular shape.
Still another type of charged particle tool, known as an e-beam spectroscopy tool, uses a focused electrical beam to study local properties on sample surface by exciting the sample surface and generating secondary particles whose characteristics are measured in some way e.g. electrons in Auger spectroscopy or Xrays-photons in Energy Dispersive Spectroscopy, etc.
Since the embodiments described herein are for a defect inspection tool, a further background of conventional defect inspection tools is provided. E-beam defect inspection tools are used in two modes. In a first mode, physical defect inspection, the electron beam gathers images of large enough areas to be able to capture a physical defect or abnormality of interest i.e. the defect physically appears in the area being imaged and is visible in the image created in the detector. Note that the defect need not be “clearly” visible for the inspection tool to operate. It must only generate a signal strong enough to suggest that a defect exists. Once the inspector has localized the defect it is typically used to gather higher resolution images in a Review SEM, as mentioned above. In a second mode, voltage contrast inspection, changes in potential at the wafer surface are detected. The change in wafer potential may happen as a result of a “physical defect” such as a particle or a purely electrical defect such as a dislocation in a crystal causing higher electrical leakage. In either case the e-beam defect inspection tool is sensing the voltage change at specific location on the semiconductor wafer as the proxy for the defect itself. The voltage change resulting from the defect typically requires some type of a excitation of the circuit underneath. This can happen as a result of the e-beam that is being used to sense the voltage contrast itself (also known as passive voltage contrast) or application of a separate electrical bias on the semiconductor wafer (also known as active voltage contrast).
One example of a conventional active voltage contrast e-beam inspection tool is provided by U.S. Pat. No. 7,679,083 B2 (“Semiconductor integrated test structures for electron beam inspection of semiconductor wafers”) to S. Jansen, et al. The '083 patent describes conventional electron beam inspection, using an electron beam that irradiates the target region, thus causing the emission of secondary electrons and a secondary electron detector measures the intensity of the secondary electron emission along the scan path of the electron beam. As a region is scanned, electrons from the electron beam induce surface voltages that vary over the scanned region due to differential charge accumulation of the irradiated features. Voltage contrast inspection operates on the principle that differences in the induced surface voltages over a scanned region will cause differences in secondary electron emission intensities.
As taught, in general, for a given feature, the intensity of secondary electron emission will vary depending on, e.g., the landing energy of the beam electrons (primary electrons) and material composition of the feature. For a given material, a secondary electron yield is a measure of a ratio of secondary electron emission to impinging primary electrons as a function of landing energy (eV). Different materials irradiated by an electron beams tuned to a specific landing energy will emit different intensities of secondary electrons. The different features within the scanned target region will be displayed in an SEM image with different grayscale shades depending on the intensity of secondary electron emission. The irradiated features having a higher intensity of secondary electron emission may be displayed brighter in an SEM image than those irradiated features having a lower intensity of secondary electron emission.
E-beam inspection tools operate by taking “images” of the semiconductor wafer at high enough resolution. The images are 20 gathered in the areas where the defect must be localized (also known as a “care area”) one of two ways. This is also illustrated in FIG. 1. Each point of the 20 image is referred to as a pixel.
1. “Step and scan”: The wafer is held stationary to capture an image of the wafer at one location. The process is repeated until the whole care area is covered.
2. “Swathing”. The wafer is moving when the image is being captured so that a whole strip of 2-D image is created also known as a swath. The process is repeated with multiple swaths until the whole care area has been covered
One common theme in both the above methods is that the care areas are sampled as full 20 images. The dwell time at each pixel is held constant at each pixel when gathering the image. Once an e-beam inspection tool has gathered an image of the care areas, it must find the defect. This is conventionally done is one of the following ways:
Array mode detection: Here the image is gathered in an area which has a repeating pattern such as a SRAM memory block. With the image, images of the neighboring memory blocks are compared and differences are flagged as a defect.
Random mode detection: Here images that have been gathered from identical dies of the wafer are compared to each other and differences are flagged as a defect. Note that the dies do need a repeating pattern inside as is required for array more inspection.
Die-to data base inspection: Here the images gathered are compared to a preexisting image saved on the computer and differences are flagged as a defect. The preexisting image may be created artificially from simulation of the inspected areas or from an image of a “golden die” that has been measured prior.
While a conventional e-beam inspection tool produces useful results, they are still less than ideal.